Chapter-2 Drilling Fluids: Barite
Chapter-2 Drilling Fluids: Barite
Chapter-2 Drilling Fluids: Barite
DRILLING FLUIDS
Drilling fluid -mud - is usually a mixture of water, clay, weighing material and
a few chemicals. Sometimes oil may be used instead of water, or oil added to the
water to give the mud certain desirable properties. Drilling fluid is used to raise
the cuttings made by the bit and lift them to the surface for disposal. But equally
important, it also provides a means of keeping underground pressures in check. The
heavier or denser the mud, is the more pressure it exerts. So weighing materials -
barite - are added to the mud to make it exert as much pressure as needed to
contain formation pressures. The equipment in the circulating system consists of a
large number of items. The mud pump takes in mud from the mud pits and sends it
out a discharge line to a standpipe. The standpipe is a steel pipe mounted vertically
on one leg of the mast or derrick. The mud is pumped up the standpipe and into a
flexible, very strong, reinforced rubber hose called the rotary hose or kelly hose.
The rotary hose is connected to the swivel. The mud enters the swivel the swivel:
goes down the kelly, drill pipe and drill collars and exist at the bit. It then does a
sharp U-turn and heads back up the hole in the annulus. The annulus is the space
between the outside of the drill string and wall of the hole. Finally the mud leaves
the hole through a steel pipe called the mud return line and falls over a vibrating,
screen like device called the shale shaker. Agitators installed on the mud pits help
maintain a uniform mixture of liquids and solids in the mud. If any fine silt or sand
is being drilled, then devices called desilters or desanders may be added. Another
auxiliary in the mud system is a device called degasser.
C-It also:
6. Supports the weight of pipe and casing
7. Serves as a medium for formation logging
Removal of Cuttings
The removal of cuttings from the face of the well bore is still one of the
most important functions of drilling fluids. Fluid flowing from the bit nozzles
exerts a jetting action that keeps the face of the hole and edge of the bit clear of
cuttings. This insures longer bit life and greater efficiency in drilling.
The circulating fluid rising from the bottom of the well bore carries the
cuttings toward the surface. Under the influence of gravity the cuttings tend to
sink through the ascending fluid; but by circulating a sufficient volume of mud fast
enough to overcome this effect, the cuttings are brought to the surface. The
effectiveness of mud in removing the cuttings from the hole depends on several
factors.
Velocity is the rate at which mud circulates, and the annular velocity is an
important factor in transporting the cuttings to the surface. Annular velocities
between 30-60 m/min. are frequently used. Velocity is dependent upon pump
capacity, pump speed, bore hole size and drill pipe size. Calculations for annular
velocity are made as follows:
Annular Velocity (m/min) = [(Pump output (m3/min.) / Annular Volume (m3/m)]
Density is weight, per unit volume of mud and has a buoyant effect upon the
particles. Increasing mud density increases its carrying capacity both by buoyancy
and particles due to additional solids in interference.
Viscosity is significant in affecting the lifting power of mud. Viscosity
depends upon the concentration, quality, and dispersal of the suspended solids. In
the field it is measured as a timed rate of flow using a Marsh funnel. Viscosity is
also measured with the Fann viscometer. These instruments are valuable aids in
measuring the effectiveness of drilling mud controls, as shown by viscosity changes
in pilot tests.
Wall Building
A good drilling fluid should deposit a good filter cake on the wall of the
hole to consolidate the formation and to retard the passage of fluid into the
formation. This property of the mud is improved by increasing the colloidal
fraction of the mud by adding bentonite and chemically treating the mud to
improve de-flocculation and solids distribution. In many cases it may be
necessary to add starch or other fluid loss control additives to reduce the fluid
loss.
gravity material, such as barite, to increase the hydrostatic head of the mud
column. The density of mud is measured with a mud balance in g/cm3, lb/gal, lb/cu
ft or psi/1000 ft of depth. The hydrostatic pressure that a column of mud exerts
upon any point in the hole can be calculated as follows:
Hydrostatic pressure:
argillaceous strata. Subsequently, it became the usual practice to add surface clays
to water in order to prepare a mud for circulation. As the primary function of a
drilling fluid is to remove cuttings, a thick slurry facilitated this action as
compared to a liquid of low viscosity and devoid of shear strength, i.e., water. Also,
clay-in-water suspensions served to keep the formation fluids confined to their
respective formations during drilling operations.
Swelling of clays
Some clays do not swell upon hydration, e.g., kaolinite clay exhibits little or no
swelling on hydration. Sodium-montmorillonite, on the other hand, swells in water to
many times its dry volume. Swelling properties of different clays are a function of
-Structure
-Chemical composition
-The amount and types of exchangeable cations
In swelling due to hydration, there are two types of swelling; swelling due to the
expansion of the crystal lattice itself, and swelling due to the adsorption of water
on surface of the clays particles.
b) Calcium Muds: Calcium muds are superior to fresh water muds when drilling
massive sections of gypsum and anhydrite as they are susceptible to calcium
contamination. When calcium is added to a suspension of water and bentonite, the
calcium cations will replace the sodium cations on the clay plates. When calcium
mud comes into contact with shaly formations, the swelling of shale is greatly
reduced in the presence of calcium cations. The major advantage of calcium mud is
their ability to tolerate a high concentration of drilled solids without these
affecting the viscosity of mud. Calcium muds are classified according to the
percentage of soluble calcium in the mud.
1- Lime Mud which contains up to 120 ppm of soluble calcium and it is prepared by
mixing bentonite, lime [Ca(OH)2], thinner, caustic soda and filtration control agent.
2- Gyp Mud which contains up to 1200 ppm of soluble calcium. It is similar to lime
mud except that the lime is replaced by gypsum and they have higher temperature
stability.
The following specifications are used as guidelines for the selection of oil:
-Specific weight (API gravity) For viscosity purposes
-Flash point It is the temperature at which oil vapor ignites upon passing flame
over the hot oil
-Fire point It is the temperature at which continuous fire is sustained over the oil
surface when flame is passed over it
Although presence of water is not required in oil muds, some water is
generally added to react with chemical additives in order to enhance the
rheological properties and plastering characteristics of oil. A number of bodying
agents have been used in oil muds to achieve the desired rheological and filtration
loss characteristics. Bodying agents can be classified into two groups:
-Colloidal size materials
-High molecular weight metal soaps
Asphalts which are colloidal-size organophilic materials are used in oil mud have to
impart required properties and control fluid loss, mainly through their absorptive
characteristics. Asphalt work with the same principle as clays in water-based muds.
Heavy metal soaps of fatty acids (emulsifiers) are added to the oil muds in order
to emulsify the water in oil. The functions of emulsifiers in oil muds are as follows:
-Imparting weak gel strength to oil muds because gel strength is necessary for
suspension of weighting materials,
-Emulsification of any water picked up during drilling operation,
-Controlling
the
tightness
of
any
water
emulsion
resulting
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Aerated Muds
Interest in under balanced drilling is increasing worldwide. In under balanced
drilling operations, pressure of the drilling fluid in the borehole is intentionally
maintained below the formation pore fluid pressure, in the open hole section of the
well. As a result formation fluids flow into the well when a permeable formation is
penetrated during under balanced drilling. Usually, aerated fluids are used in under
balanced drilling operations. Most frequently used aerated fluids are air-liquid
mixtures, foams, mist and gas.
Selection of Drilling Fluids
Selection of the best fluid to meet anticipated conditions will minimize well
costs and reduce the risk of catastrophes such as stuck drill pipe, loss of
circulation, gas kick, etc. Consideration must also be given to obtain adequate
formation evaluation and maximum productivity. Some important considerations
affecting the choice of muds to meet specific conditions are presented as follows:
-Mud-making shales : Thick shale sections containing dispersible clays cause a rapid
rise in viscosity as cuttings become incorporated in the mud. When the mud is unweighted, it is easy to reduce the excessive viscosity, however, when the mud is
weighted, costly chemicals such as barite should be used to restore the mud
properties.
-High temperature : Most of the mud additives degrade with time and elevated
temperatures, which are higher than degradation temperature. Special additives
must be used to make mud resistive to high temperatures.
-Hole instability : Two basic forms of hole instability are hole contraction and hole
enlargement. If the lateral earth stresses bearing on the walls of the hole exceed
the yield strength of the formation, hole slowly contracts. He density of the mud
should be high enough to resist contracting. Hole enlargement occurs at watersensitive shale zones. Shale stabilizers should be used to prevent hole enlargement.
-Rock salt : To prevent the salt from dissolving and consequently enlarging the hole,
either an oil base mud or a saturated brine must be used.
-Hole inclination : In highly deviated holes, torque and drag are a problem because
the pipe lies against the low side of the hole and the risk of pipe stuck is high.
Proper muds should be selected to prevent such problems, and keep cutting to be
removed from the well properly.
Formation evaluation : The selected mud should be suitable for logging tools, MWD,
Productivity impairment : Solids control or density adjustments should be
considered properly to keep the formations non-damaged or blocked.
Viscosity: The Marsh funnel viscosity is used for routine field measurement.
Report viscosity in seconds per quart. Plastic viscosity (cP) and Yield Point
(lb/100 sq ft) are measured with the Fann viscometer.
Rheological Properties:
Determine viscometer readings to calculate the following for a drilling or
completion/ work over fluid: Plastic Viscosity (PV, cp); Yield Point (YP, lbf/100 ft2);
Gel Strength - Max. dial reading at 3 rpm- (Tau, lbf/100 ft2); Apparent viscosity
(AV, cp); Consistency index (K, lbf/secn/cm2); Yield stress (YS, lbf/100 ft2); Flow
index (n, unit-less)
PV = 600 - 300
YP = 600 - PV
AV = 600 / 2
n = 3.32 log (600 / 300)
K = 300 / (511n)
Gel strength = Max. dial reading at 3 rpm
Example:
Given the following well data, determine PV, YP, AV, n and K.
600 = 36
300 = 24
Solution:
PV = 36 -24 = 12 cP
YP = 24 12 = 12 lbf/100 ft2
AV = 600 / 2 = 36 / 2 = 18 cP
n = 3.32 log (600 / 300) = 3.32 log (36/24) = 0.5846
K = 300 / (511n) = 24 / 5110.5846 = 0.626
Drilling Fluid Additives
There are fundamental aspects that have to be controlled in order to have an
effectively and successfully purposing drilling fluid. These aspects can be
categorized as:
-Viscosity Control
-Fluid Loss Control
-Weight Control
-Corrosion Control
Viscosity Control
Viscosifiers :Many different products are classified as viscosifiers. Bentonite,
attapulgite clays, subbentonites and polymers are most widely used viscosity
builders. Bentonite, attapulgite clays and sub-bentonites all form colloidal
suspensions in water. They increase viscosity, yield point and gel-strength by intersurface friction and by
Phosphates :
-Useful as effective thinners in most bentonite water-based muds at shallow
depths,
-Small amounts of thinner are very effective at temperatures less than 130F
-pH is around 5, so caustic soda or some other hydroxyl ion containing additive is
required to maintain pH above 7.0,
-They have very low temperature stability,
-They have no fluid loss ability,
-At relatively low temperatures, they revert to orthophosphates, which severly
flocculates clays and increase viscosities and gel strengths,
Lignites :
-They have a temperature stability of 400F,
-They are organic thinners serving both as dispersants and as fluid loss control
agents due to their colloidal structure,
-They are not suitable for high-salt content fluids due to the insolubility of lignite
in salt,
-They may cause disintegration of active solids,
Tannins :
-They are dual-purpose materials serving as dispersants and fluid loss control
agents,
-They are effective in thinning lime muds and cement contaminated muds,
Lignosulfonates :
-They are effective for lime muds,
-They are effective as general purpose thinners due to the heavy metal-ions
attached,
-They have temperature stability in the range of 300F to 350F,
-They are dual-purpose additives serving as both dispersants and fluid loss
additives,
-They may cause disintegration,
Bentonite :
-A multipurpose additive that aids in fluid loss control, barite suspension, viscosity
generator for hole cleaning purposes,
-It is not suitable for use in environments high in concentration of sodium, calcium
or potassium without pre-hydration,
-It may contaminate formations such as salt or anhydrite,
-Slurries are susceptible to the effect of high temperature gelation which could
cause an increase in the fluid loss,
Starch :
-They work well as fluid-loss agents in the presence of low soluble calcium or sodium
ions,
-They are suitable for salt-water or gyp muds,
-An increase in viscosity is observed when it is used,
-A bactericide must be used to prevent digradation and fermantation,
-It degrades at temperatures over 200F,
CMC :
-It is active in low to moderate contaminating-concentrations, which makes it
suitable for use in inhibited muds,
-It has a temperature stability up to 400F,
-A thinner may be necessary to counteract the viscosity effects of the additive,
-It may cause a thinning effect in some salt muds
Cypan :
-It can be used successfully in high-temperature regions due to its stability up to
400F,
Weighting Materials
They are substances with high specific gravity which can be added to the mud to
increase its density, usually to control formation pressure. Barite is by far the
most common weighting material used in drilling fluids. It has an API defined
specific gravity of 4.2, which makes it possible to increase mud weight up to 21 ppg.
It is cheap and readily available. However, suspension of barite requires high gel
strength and viscosity. Hematite is sometimes used depending on the availability.
Calcium Carbonate is an additive used in drilling muds, workover fluids and packer
fluids to increase the fluid density. It has a specific gravity of 2.7, therefore, the
fluid density can be increased up tp 12 ppg. It is more economical than other
agents, and can be suspended easier than barite. Also it is acid soluble.
Lost Circulation Materials
-Fibous materials for seepage losses and in combination with other materials
If two substances having different densities are mixed then the density of
the mixture is a function of the quantity and density of the components of the
mixture. This relationship can be expressed as follows:
V1D1 + V2D2 = (V1 + V2) (DR)
where,V1=Volume of the first substance; V2=Volume of the second substance;
D1=Density of the first substance; D2=Density of the second substance; DR =
Density of the resulting mixture.
Increasing the weight of drilling mud with a material such as barite can be
related in a similar manner.
V1W1 + V2WB = (V1 + V2) (W2)
Example-1
How much barite is required to increase the density of 300 bbls of mud from 4 ppg
to 15 ppg.
Solution:
Sacks of barite / 100 bbl of mud = [1490 (W2 W1)] / (35.4 W2)
Sacks of barite / 100 bbl of mud = [1490 (15 14)] / (35.4 15) = 72.4
Sacks of barite / 300 bbl of mud = 72.4 x 3 = 217.2
The below formula is called the starting volume formula. It is used to
determine the initial volume of mud to start with in order to obtain a specific
volume of mud after weighting.
SV = [(35.4 W2) / (35.4 W1)] DV
Example-2 How much 12 ppg mud is needed to prepare exactly 250 bbls of 14 ppg
mud?
Solution:
SV = [(35.4 W2) / (35.4 W1)] DV
SV = [(35.4 14) / (35.4 12)] 250 = 228 bbls
The below formula is called the starting volume formula. It is used to
determine the initial volume of mud to start with in order to obtain a specific
volume of mud after weighting.
% by volume water = 100 [(W1 W2) / (W2 8.33)]
Example-3 What volume of water will be necessary to reduce the density of 1200
bbls of 15.2 ppg mud to 13.5 ppg?
Solution:
% by volume water = [(15.2 13.5) / (13.5 8.33)] = 0.327
bbls of water to add = 1200 x 0.327 = 392
The above problem can be solved similarly to obtain the result directly in barrels
rather than percent by volume.
bbl of water to be added = present vol. [(W1 W2) / (W2 8.33)]
bbl of water to be added = 1200 [(15.2 13.5) / (13.5 8.33)] = 392
Example-4 Calculate how much water and barite must be mixed to make exactly
500 bbl of 14 ppg mud?
Solution:
V1W1 + V2W2 = VFWF
V1 + V2 = VF and V2 = VF V1
V1 (8.33) + V2 (35.4) = 500 (14)
V1 (8.33) + (500- V1) 35.4 = 7000
V1 = 395 bbl of water
V2 = VF V1
V2 = 500 395
V2 = 105 bbl of barite
105 (14.9) = 1565 sx of barite
Example-5 Calculate how much oil, water and barite are required to make exactly
300 bbl of 15 ppg oil mud with 80/20 oil/water ratio.
Solution:
V1W1 + V2W2 + V3W3 = VFWF
V1 + V2 +V3 = VF
The oil and water have a ratio of 80/20 so that they can be considered as one
volume (V) that is 80 % oil and 20 % water.
V1 + V2 +V3 = VF
V + V3 = 300
V3 = 300 - V
V[ (0.8) (6.8) + (0.2) (8.33)] + V3 (35.4) = 300 (15)
V[ (0.8) (6.8) + (0.2) (8.33)] + (300 V) (35.4) = 300 (15)
Example-6 Calculate how much oil will have to be added to change the oil/water
ratio of 100 bbl of 80/20 oil mud to 90/10.
Retort Analysis: Oil : 64 %, Water : 16 % and Solids: 20 %
Solution:
The oil water ratio is changed by the following formula:
{Volume of water / [(Volume of water + Pressure volume of oil + Volume of oil to be
added) ]} = New percent of water in liquid phase
Volume of water = 100 x (0.16) = 16 bbl
Volume of oil = 100 x (0.64) = 64 bbl
[16 / (16 + 64 +V)] = 0.1
V = 80 bbl
Example-7 If the same 80/20 mud is to be changed to 75/25 oil water ratio,
calculate how much water will have to be added
Retort Analysis: Oil : 64 %, Water : 16 % and Solids: 20 %
Solution:
The oil water ratio is changed by the following formula:
{Volume of oil / [(Volume of oil + Pressure volume of water + Volume of water to be
added) ]} = New percent of oil in liquid phase
Volume of water = 100 x (0.16) = 16 bbl
Volume of oil = 100 x (0.64) = 64 bbl
[64 / (16 + 64 +V)] = 0.75 = 5 bbl